1. Field of the Invention
The present invention relates to a solid oxide electrolyte fuel cell and to a method for fabricating the same. More particularly, the present invention relates to a solid oxide electrolyte type fuel cell in which a solid oxide electrolyte and an anode plate are matched with respect to their coefficients of thermal expansion and to a method for fabricating such a fuel cell.
2. Description of Related Arts
Generally, fuel cells having a solid oxide electrolyte such as zirconia are operated at a high temperature as high as 800.degree. to 1,100.degree. C. and hence they have various features. For example, their power generation efficiency is high, they require no catalyst, and they are easy to handle because the electrolyte used is solid. Therefore, solid oxide electrolyte fuel cells are viewed as third generation fuel cells.
However, since they are composed mainly of ceramics, the solid oxide electrolyte fuel cells tend to fail thermally, and there is no appropriate means for sealing gases. As a result it has been difficult to realize an acceptable model.
A conventional approach uses a unit cell of a unique form, i.e., a tubular form in order to overcome the aforementioned two problems. This test operation was carried out successfully (cf. 1988 IECEC Proceedings, Vol. 2, p. 218 (1988)). However, power generation density per unit volume of fuel cell is still low and there is no anticipation that an economically advantageous fuel cell can be obtained.
In order to increase the power generation density of a solid oxide electrolyte fuel cell, it is necessary to construct it in a flat plate form. A planar solid oxide electrolyte fuel cell includes a solid electrolyte (dense), an air electrode (porous) on one side of and a fuel electrode (porous) on the other side of the solid electrolyte, and two bipolar plates sandwiching therebetween the air electrode and the fuel electrode, the respective bipolar plates having grooves on both sides thereof for passing reaction gases (cf. 1988 IECEC Proceedings, Vol. 2, p. 218 (1988)).
FIG. 1 is an exploded perspective view showing another type of conventional solid oxide fuel cell. As shown in FIG. 1, a conventional planar solid oxide fuel cell 10 includes an anode plate 12 which has thereon a solid oxide electrolyte element 11, a cathode 13, a cathode plate 14, and a separator 15, superimposed one on another in this order. The anode and cathode plates 12 and 14, respectively, are formed with grooves, and the anode grooves 16 and cathode grooves 17 are perpendicular to each other, and different gases pass through the two groups of grooves 16 and 17.
Conventionally, the anode plate 12 is made of a porous cermet composed of zirconia stabilized with yttria, and the solid oxide electrolyte element 11 is made of zirconia stabilized with yttria. The conventional anode plate 12 has a coefficient of thermal expansion of 12.times.10.sup.-6 /.degree. C. to 14.times.10.sup.-6 /.degree. C., and the solid oxide electrolyte element 11 has a coefficient of thermal expansion of 10.5.times.10.sup.-6 /.degree. C.
This difference in coefficient of thermal expansion between the anode plate 12 and the solid oxide electrolyte element 11 results in the occurrence of curls or cracks in the anode plate 12 and also cracks in the solid oxide electrolyte element 11. This causes cross leaking between a fuel gas and an oxidizer gas, or contact failure which decreases the performance of the fuel cell.